yes! sure :)
Suppose prefix of SER-B is 123 and the rest of URI is some number (e.g. sip:1235555@blah-blah.net ), then in your INVITE route:
.....
sl_send_reply("100", "Trying");
# Check if From SIP URI is not from our domain, but has prefix 123 if (!check_from() && !search("^From:*sip:123[0-9]*@") ) { if (!www_authorize("","subscriber")) { www_challenge("","1"); break; }; };
.....
This is too simple regular expression for search() function, i think you'll need more complicated than this. :(Im afraid this one might match not just 123 prefix but anything else... Try this anyway. See if that works.